1. Field of the Invention
This invention relates to electronic circuits for energizing a load. In particular, it relates to such a circuit used in conjunction with a sensing element such as a proximity switch.
2. Prior Art
Circuits are known in which a constant current or current-limiting subcircuit is employed and is adapted to be coupled to a proximity detector and to a load which is energized in response to the detection of predetermined metallic components by the proximity detector. One such circuit is shown in the Buck U.S. Pat. No. 3,932,803 in which a constant current source is in series with a voltage breakdown device. That circuit uses a single SCR which fires to energize a load when metal is detected yet maintains a sufficient voltage across the breakdown device to keep the detector's oscillator-amplifier continuously operative. In the Buck circuit there is a flow of current through the Zener diode as well as through the SCR and its associated components causing an unnecessary second voltage drop which reduces the electrical efficiency of the circuit. Buck's circuit has the SCR in series with the voltage breakdown device. Another U.S. Pat. No. 4,135,124 by Buck also calls for an SCR in series with a breakdown device and therefore has two voltage drops in the same branch. The same is true with respect to Buck U.S. Pat. Nos. 3,932,774 and 4,110,678. Buck U.S. Pat. No. 4,100,479 discloses a constant current source and a circuit which includes two SCRs, the second one being substantially across the constant current source and across the DC supply source. However, that circuit requires two Zener diodes in addition to two SCRs and is therefore more complex and possibly more expensive than the one illustrated in this application.
It is therefore among the objects of the present invention to provide a circuit which:
1. Enables a greater current energization of a load circuit than those known in the prior art. PA1 2. Requires only a single SCR and a single Zener diode. PA1 3. Has only a single voltage drop across the DC supply circuit and the constant current source when its SCR is conductive. PA1 4. Has other advantages as will be explained below.